Environmental pollution concerns have led those in the coatings and resins arts to develop curable compositions from which organic petroleum based solvents have been eliminated or at least substantially reduced. Additionally, the high cost and scarcity of petroleum by-products has generated interest in the development of curable compositions based entirely, or at least substantially, on inorganic components.
Inorganic coating compositions based on alkali metal silicates and inorganic phosphate hardeners are known. However, when alkali metal silicates and inorganic phosphate hardeners are mixed, they tend to rapidly coagulate, solidify or the like due to the strong reactivity between the silicate and the phosphate. The particles or small lumps formed in these alkali silicate/phosphate hardener compositions, due to this tendency toward prematurely rapid reaction are undesirable, for example, in applications employing spraying techniques. Additionally, cured coatings produced from compositions containing such particles contain undesirable lumps.
A method is described in U.S. Pat. No. 3,930,876 directed to preventing the rapid reaction of the silicate with the phosphate to increase pot life wherein a condensed phosphate hardener prepared using double-oxides having the "spinel" structure is pretreated with a silicate or a modified silicate to form a slurry or paste having a pH of 5 to 9.5, preferably 6 to 8.5. The procedure described therein requires that the condensed phosphate hardener be prepared using a double oxide having a "spinel" structure. The pretreatment of the hardener described in U.S. Pat. No. 3,930,876 generally takes from 10 to 30 hours and produces an inert coating on the phosphate hardener containing no free acid rendering it less reactive.
The present invention is directed to curable inorganic coating compositions based on water-soluble and/or water-dispersible alkali metal silicates which are relatively storage stable, essentially lump-free, easily sprayable, and curable at relatively low temperatures to smooth, essentially lump-free, durable coatings which are resistant to high temperatures, water, detergents and cracking. The invention is also directed to a method for producing the compositions of the invention and to a method of forming coatings from the compositions. Coatings produced from the compositions of the invention are suitable for use, for example, as a replacement for porcelain in appliances. It is notable that coatings prepared from the compositions of the present invention are substantially less energy intensive than porcelain coatings and essentially lump free as compared to coatings prepared from aqueous silicate/condensed phosphate hardener compositions generally known in the art.